Mercedes Benz has just released the OM651, a new
four cylinder diesel engine with no less than 150kW and a massive 500Nm – from
just 2.14 litres!
In the C-class 250 CDI BlueEFFICIENCY Prime Edition sedan, the
NEDC fuel economy figure is just 5.2 litres/100km. The 0-100 km/h time for the
1645kg car is a claimed 7.0 seconds.
Gaining outputs of 70 kW/litre and 186 Nm/litre
requires a range of sophisticated technologies. Let’s take a look.
The engine uses two series turbochargers
integrated into a single compact module. These turbos comprise a small high
pressure turbo and a larger low pressure turbo.
The high pressure turbine has a diameter of 38.5mm
and is positioned directly in the exhaust manifold. The exhaust gases flow
through this turbine first, causing it to rotate at speeds of up to 248,000 rpm.
Integrated into this turbine housing is a bypass duct, which can be opened or
closed by means of a charge-pressure control flap triggered by an actuator.
If the duct is closed, the whole exhaust stream
flows through the high pressure turbine, allowing charge pressure to be built up
at low engine revs.
As the engine speed increases, the charge-pressure
control flap opens to prevent the high pressure turbocharger from becoming
overloaded. A portion of the exhaust stream then flows through the bypass duct.
Downstream from the high pressure turbine, the two exhaust gas streams join up
again, and any remaining exhaust energy drives the 50mm low pressure turbine at
a maximum speed of 185,000 revolutions per minute. The low pressure turbine uses
a wastegate for exhaust gas bypassing.
Once the engine reaches medium revs, the high
pressure turbine's charge pressure control flap is opened so wide that the high
pressure turbine ceases to perform any appreciable work. This allows the full
exhaust energy to be directed with low losses into the low pressure turbine,
which then does all of the turbine work.
The two compressors are likewise connected in
series and are in addition connected to a bypass duct.
The combustion air from the air cleaner first
flows through the low pressure compressor (diameter 56.1 mm) where it is
compressed as a function of the low pressure turbine's operating energy input.
This pre-compressed air now passes into the high pressure compressor (diameter
41 mm) that is coupled to the high pressure turbine, where it undergoes further
compression – the result is a genuine two-stage turbocharging process.
Once the engine reaches medium revs, the high
pressure compressor can no longer handle the flow of air, meaning that the
combustion air would heat up too much. To avoid this, the bypass duct opens to
carry the combustion air past the high pressure compressor and directly to the
intercooler for cooling. In this case, the charge pressure control flap is
completely open too, meaning that the high pressure turbine is no longer
performing any work. This is the equivalent of single-stage turbocharging.
Intercooler and EGR Cooler
A front-mount intercooler is fitted to reduce
intake air temperatures from as high as 140 degrees C after the air leaves the
After the intercooler, an electrically controlled
valve ensures precise regulation of the fresh air and recirculated exhaust gas.
So as to optimise the quantity of exhaust gas recirculated and thereby achieve
high recirculation rates, the exhaust gases are cooled down as required in a
powerful heat exchanger with a large cross-sectional area.
Variable Intake Manifold
The combustion air subsequently flows into the
intake manifold that supplies air to each cylinder in a uniform manner. Built
into the manifold is an electrically controlled intake port shut-off which
allows the cross-sectional area of each cylinder's intake port to be smoothly
reduced in size. This alters the swirl of the combustion air in such a way as to
guarantee that the charge movement in the cylinders is set for optimum
combustion and exhaust emissions over the full range of engine loads and rev
Rear Cam Drive
So as to allow appropriate pedestrian impact
requirements to be met when the engine is installed longitudinally, a
rear-mounted camshaft drive is used.
The valve timing mechanism is another new
development and reduces friction at the 16 intake and exhaust valves, which are
controlled by one overhead intake shaft and one overhead exhaust shaft acting
via cam followers featuring hydraulic valve clearance compensation. The
camshaft, Lanchester balancer and the ancillary assemblies are driven by a
combination of gearwheels and just a very short chain drive.
Fourth Generation Common Rail Injection
Fourth-generation common-rail technology is used.
Rail pressure has been increased by 400 bar to 2000 bar, and new piezoelectric
injectors featuring direct injector needle control allow more flexible injection
timing. The maximum ignition pressure is 200 bar which also contributes to the
high output. Twin airflow meters are used.
The new four-cylinder diesel engine will replace
four of the existing diesel engines at Mercedes-Benz. The engine can be
installed either longitudinally or transversely. Mercedes-Benz is expected to
produce up to 700,000 units per year.
Footnote: No CO2 emissions figures have
been released by Mercedes Benz.
you blinked when you saw that 233 Nm/litre figure, you won’t be alone in your
surprise. This Mercedes Benz chart shows the evolution of specific power and
torque outputs through their diesel engine progression – over the naturally
aspirated OM601 series, the latest engine has nearly 3.8 times the specific
torque and 2.6 times the specific power!